Excess Properties of Aqueous Dilutions of Ammonium- and Phosphonium-Based Deep Eutectic Solvents

This work focused first on the measurement at 25 °C and ambient pressure of the thermal conductivities (k), thermal diffusivities (α), and heat capacities (Cp) of aqueous dilutions of two deep eutectic solvents (DESs). The first one composed of choline chloride (ChCl) and urea (U) in a 1:2 molar ratio (ChCl:2U), and the second one composed of methyltriphenylphosphoniumbromide (M3PhPBr) and glycerol (Gly) in a 1:3 molar ratio (M3PhPBr:3Gly). Then, we compared the deviation from ideality of the thermal conductivity (Δk) with those experienced by other excess properties, including excess molar volume (VE), excess viscosity (ln ηE), and excess adiabatic compressibility (ΔβS). The aim was finding some correlation among them in the sign of the deviation and/or the specific DES content at which the maximum deviation from ideality is reached. Moreover, we have compared the hypersonic velocities (νH) obtained from Brillouin spectroscopy with regular ultrasonic velocities (νU) to identify the DES content for which, in terms of relaxation, the mixture moves from a purely dissipative fluid to an “elastic medium”. Finally, we also investigated whether 1H NMR spectroscopy could also be used to identify the DES content for which the deviation of ideality was the maximum.